388 PHYLUM ARTHROPODA 



glide with the serpents, and they swim with the fish." 

 They beat the elastic air with their wings, and though there 

 cannot be so much complexity of movement as in birds 

 where the individual feathers move, the insect wing is no 

 rigid plate, and its up-and-down motions are complex. 

 They can soar rapidly, but their lightness often makes 

 horizontal steering difficult. The wind often helps as well 

 as hinders them ; thus the insects which fly in and out 

 of the windows of express trains are probably in part 

 sucked along. Marey calculates the approximate number 

 of wing strokes per second at 330 for the fly, 240 for the 

 humble-bee, 190 for the hive-bee, no for the wasp, 28 

 for the dragon-fly, 9 for a butterfly. For short distances 

 a bee can outfly a pigeon. 



Skin. — As in other Arthropods, the epidermis (or hypo- 

 dermis) of Insects forms a firm cuticle of chitin, which in 

 the exigencies of growth has sometimes to be moulted. 

 This cuticle is often finely marked, so that the animal seems 

 iridescent ; and there are many diff"erent kinds of scales, 

 hairs, and spines. Chitin is not favourable to the develop- 

 ment of skin glands. Most insects have '* salivary glands " 

 opening in or near the mouth. Bees have wax-making 

 glands opening on the abdomen ; aphides have glandular 

 tubes ; not a few have poison bags ; and many larvae 

 besides silkworms have organs from which are exuded the 

 threads of which a cocoon is made. 



Muscular system. — In very active animals like Insects, 

 we of course find a highly developed set of rapidly contract- 

 ing striped muscles. These work the wings, the legs, and 

 the jaws. The resulting movements have this further 

 significance, that they help in the respiratory interchange of 

 gases, and in the circulation of the blood. 



Nervous system. — As in other Arthropods, the nervous 

 system consists — (a) of a dorsal brain or supra-cesophageal 

 ganglionic mass, and {b) of a double ventral nerve-cord 

 with a number of paired ganglia, of which the most anterior 

 (the sub-cesophageal) are linked to the brain by a ring com- 

 missure around the gullet ; and (c) of nerves given off from 

 the various ganglia to the sense organs, the alimentary 

 canal, and the other organs. In many of the higher in- 

 sects the ganglia of the ventral nerve-cord are in some 



